After reviewing the conditions described in this situation, several comments are appropriate.  The mixing intensity at 120 rpm appears to be adequate for minimal fluid motion and liquid blending.  Because of the small size of the container and the general description, this mixing review assumed that the tank did not have baffles and the viscosity of the slurry or solution was low.  Without baffles, most of the liquid motion will be solid body rotation and provide little vertical mixing with a strong tendency for a surface vortex to form.  While the active mixing will not be particularly effective, the small size should still accomplish rapid uniformity.
 
At 120 rpm, the estimated impeller power is only 0.199 Watts or 0.0003 Hp, so a 1/2 hp motor is much more than necessary.  The 1N impeller force is unclear as to direction or effect.  The impeller torque is only 0.016 Nm.  Of course the mixing intensity can be increased with a higher rotational speed, such as 150 rpm, provided surface vortexing does not become so great as to draw air into the blend or splash liquid out of the container.

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